A process for pulping a lignocellulosic material, for example, wood, straw or bagasse, by using an alkaline sulfide cooking liquor containing, as main components, sodium sulfide and sodium hydroxide at an elevated temperature, is referred to as an alkaline sulfide pulping process. This alkaline sulfide pulping process, which includes a kraft process, is a most important chemical pulping process due to its advantage in that the quality of the resultant pulp is higher than that of other pulping processes, for example, a sulfite pulping process. However, on the other hand, the conventional alkaline sulfide pulping process has two disadvantages in that the yield of the resultant pulp is relatively small and the sulfur compound used in the kraft cooking liquor causes an offensive smell. In recent years, these two disadvantages have become an increasing important problem from the standpoints of the manufacturing cost, shortage of wood resources and environmental pollution.
In order to eliminate the above-mentioned disadvantages of the conventional alkaline sulfide pulping process, various approaches were looked into for accelerating the delignification reaction between the lignocellulosic material and the pulping liquor and for preventing the decomposition of the carbohydrates in the lignocellulosic material. Examples of these approaches are: a polysulfide process [Pulping Process Sven A Rydholm, 1965, published by Interscience]; a process comprising adding sodium borohydride (U.S. Pat. No. 3,042,575), hydrazine (U.S. Pat. No. 3,161,562), hydroxylamine (U.S. Pat. No. 3,401,076) or a nitroaromatic compound [Svensk Papperstid 71 (23) 857.about.863 (1968)]; a process for pre-treating wood chips with hydrogen sulfide (U.S. Pat. No. 3,520,773), and; an alkafide process (U.S. Pat. No. 1,902,106, U.S. Pat. No. 3,347,739). However, all of the above-mentioned approaches, except for a MOXY process belonging to the polysulfide process (Japanese Patent Application Publication (Kokoku) No. 50-40395 which corresponds to U.S. Pat. No. 4,024,229, Norsk Skogindustri June, 1978, pages 134 to 140), have not yet been practically utilzed due to the fact that the approaches cause the pulping apparatus to be expensive or complicated, the cost of the pulping operation to be very high, or the processability of the pulping process to be poor, or result in an environmental pollution or exhibit a poor effect in pulping hardwood. In addition, none of the above-mentioned approaches positively avoid the use of a sulfur compound in the kraft pulping mill nor have any of them eliminated the offensive smell problem in the kraft pulping mill.
Recently, since B. Bach and G. Fiehn, Zellstoff und Papier, vol 21, No. 1, pages 3 to 7 (1972) and related East German Pat. No. 98,549 disclosed that the yield of pulp in the alkaline pulping process could be increased by adding an anthraquinone compound, particularly anthraquinone-2-monosulfonic acid salt (AMS), to the alkaline pulping liquor, various processes in which various anthraquinone compounds were used, have been developed. Examples of these processes are: a two-stage pulping process which comprises a first soda stage and second oxygen-alkali stage or a first kraft stage and second oxygen-alkali stage, and in which sodium anthraquionone-2-sulfonate (AMS) is added to the pulping liquor in the first stage (Japanese Patent Application Laying-open (Kokai) No. 50-29801, U.S. Pat. No. 3,888,727); a pulping process in which the lignocellulosic material is pre-treated with an alkali solution containing AMS (Canadian Pat. No. 986,662), and; a process in which a water-soluble oxygen carrier consisting of a quinone compound or hydroquinone compound is added to an oxygen-alkali pulping liquor (Japanese Patent Application Laying-open (Kokai) No. 51- 119801, West German Patent Application Laying-open (Offenlegungsschrift) No. 2,610,891).
Examples of processes comprising adding a quinone compound other than AMS are: a process in which a quinone compound is added to an alkaline cooking liquor (Japanese Patent Application Laying-open (Kokai) No. 51-43403); a process in which a sulphur free cyclic keto compound is added to an alkaline pulping liquor (Japanese Patent Application Laid-Open Publication (Kokai) No. 52-37803, U.S. Pat. No. 4,012,280); a process in which a quinone compound is added to a sulfite pulping liquor (Japanese Patent Application Laid-Open Publication (Kokai) No. 51-112903); a process in which a quinone or hydroquinone compound is added to an alkaline pulping liquor or a sulfite pulping liquor (Japanese Patent Application Laying-open (Kokai) No. 52-155202); a two-stage pulping process which comprises a first soda stage and second oxygen-alkali stage or a first kraft stage and second oxygen-alkali stage, and in which a quinone or hydroquinone compound is added to the pulping liquor in the second stage (Japanese Patent Application Laying-open (Kokai) No. 53-58004); a process in which a cyclic keto compound and a nitro aromatic compound are added to a soda pulping liquor (Japanese Patent Application Laying-open (Kokai) No. 53-74102, West German Patent Application Laying-open (Offengungsschrift) No. 2,755,769); a soda pulping method in which a soda cooking liquor contains both a nitro aromatic compound and a 9, 10-diketohydroanthracene compound selected from unsubstituted and lower alkyl-substituted Diels-Alder adducts of naphthoquinone and benzoquinone (Japanese Patent Application Laying-open (Kokai) No. 53-74103, U.S. Pat. No. 4,036,680); a process in which a 9,10-diketohydroanthracene compound is added to an alkaline pulping liquor (Japanese Patent Application Laying-open (Kokai) No. 53-74104, U.S. Pat. No. 4,036,681); a process in which a 9,10-dioxyhydroanthracene compound is added to an alkaline or sulfite pulping liquor (U.S. Pat. No. 4,181,565, West German Patent Application Laying-open (Offengungsschrift) No. 2,754,991), and; a process in which various quinone compounds are added to an alkaline or sulfite pulping liquor (Japanese Patent Application Laying-open (Kokai) No. 53-139803).
In the above-mentioned prior arts, the quinone or hydroquinone compound alone or a combination of the quinone or hydroquinone compound and oxygen or an oxidizing agent, such as a nitro aromatic compound, were used for accelerating the delignification reaction and increasing the yield of the resultant pulp.
The inventors studied in detail the pulping process using a cooking liquor containing a quinone compound. As a result of this study, it was found that Na.sub.2 S and NaHS in the cooking liquor are active as a reducing agent only when the cooking liquor is in a weak alkaline condition or neutral condition, and can reduce the quinone compound into the corresponding hydroquinone compound, while when the cooking liquor is in a strong alkaline condition, Na.sub.2 S and NaHS can not exhibit the reducing activity. For this reason, when the addition of the quinone compound is applied to an alkaline sulfide pulping process, an expensive quinone or hydroquinone compound should be added in a large amount. On the basis of this knowledge, the inventors have previously provided a novel alkaline sulfide pulping process which comprises cooking lignocellulosic material with a cooking liquor containing, as main components, sodium sulfide and sodium hydroxide, to which have been added a quinone or hydroquinone compound and at least one reducing assistant selected from the group consisting of sulfites, hydrogensulfites, thiosulfates and formates (Japanese Patent Application Laying-open (Kokai) No. 54-82402). Also the inventors have provided another novel alkaline sulfide pulping process in which a 9,10-diketohydroanthracene compound and/or a 9,10-dihydoxyhydroanthracene compound are added to the pulping liquor, instead of the quinone or hydroquinone compound in the above-mentioned pulping process (Japanese Patent Application Laying-open (Kokai) No. 54-106601).
The inventors further studied in detail the reducing assistant to be added to the redox system in the above-mentioned alkaline sulfide pulping processes. As a result of this study, it was found that, when the reducing assistants produced as a by-product in processes other than a cooking process in a pulping mill are added to the pulping liquor, sulphur circulating through the recovery process is increased in its total amount when the reducing assistants are the oxyacid salt of sulphur, which causes the sulfidity of the cooking liquor to be increased.
The inventors conducted earnest studies to overcome the above-mentioned problem. It is known that a quinone compound which is capable of reversibly converting to a corresponding hydroquinone compound is a catalyst for the conversion of sodium sulfide to sodium thiosulfate by oxydizing an alkaline liquor containing sodium sulfide (U.S. Pat. No. 1,855,353, Japanese Patent Application Laying-open (Kokai) No. 49-15248, Japanese Patent Application Laying-open (Kokai) No. 51-88703). On the basis of this knowledge, the inventors attempted to add a small amount of anthraquinone to a green liquor or white liquor in the kraft recovery process, aerate the liquor under mild conditions at normal temperature and pressure, and analyze the oxidized liquor with the lapse of the aerating time. Surprisingly, it was discovered that, in addition to sodium thiosulfate, a large amount of sodium sulfite is preferentially produced in the course of the air oxidation. In addition, it was also discovered that, when sulphur-containing oxyacid salts, including sodium sulfite, are produced in situ in the cooking liquor, the yield of the resultant pulp becomes higher than that in the case where sodium sulfite is added to the cooking liquor from the outside of the pulping system, and no accumulation of sulphur occurs in the pulping process and in the recovery process.
The present invention was made on the basis of the above-mentioned discoveries.